Extra embryonic yolk sac, Allantois, Amnion,Chorion.How they form, their structure and advantages

1. AMNIOTE and ANAMNIOTE
• Reptiles, birds and mammals embryos are
covered by an umbrella like covering
called amnion filled with fluid which
prevents it from dessication, they are
called AMNIOTES
• Fishes and amphibians lay eggs in water,
so no problem of dessication , so no
amnion and are called ANAMNIOTES

2. The blastoderm in amniotes ( reptiles, birds,
mammals ) not only give rise to the embryo, but
also to certain structures that lie outside the
embryo. They have evolved to enable the birds to
lay eggs on land and the eggs to develop on dry
land.
So EXTRA EMBRYONIC MEMBRANES are
those extra embryonic structures which are
formed from extra embryonic tissues but lie
outside the developing embryo
EXTRA EMBRYONIC MEMBRANES

3. During amniote development, four
extraembryonic membranes form around the
embryo:
The chorion functions in gas exchange
The amnion encloses the amniotic fluid
The yolk sac encloses the yolk
The allantois disposes of waste products and
contributes to gas exchange
EXTRA EMBRYONIC MEMBRANES
The extraembryonic membranes in mammals
are homologous to those of birds and other
reptiles and develop in a similar way

4. SOMATOPLEUR & SPLANCHNOPLEUR

5. YOLK SAC
• It is found in animals with polylecithal and
meiolecithal eggs ( reptiles, birds ,
prototherians ) with meroblastic cleavage
and yolk is not disributed into blastomeres,
so there is need to digest the yolk lying
outside.
• Yolk sac of amniotes (formed of
splanchopleur )differ from anamniotes
( formed of endo, meso, ectoderm )

6. YOLK SAC IN DIFFERENT
VERTEBRATES
• In fishes, it is present in most of
elasmobranchs ( scoliodon- viviparous fish
) and teleosts where it act as yolk sac
placenta to exchange materials between
vessels of embryo and mother.
• In amphibians, it is present in those with
polylecithal eggs like Necturus but absent
in others with mesolecithal eggs
whereyolk is evenly distributed in
blastomeres.

7. YOLK SAC IN CHICK
• -Formed of extraembryonic
splanchnopleur with endoderm inner
and splanchic mesoderm outer side.
At first , yolk sac has a wide opening
into the midgut.
• -As development proceedes ,its
passage into the midgut is reduced to
a narrow YOLK SAC STALK or
UMBILICAL STALK, whose opening is
called UMBILICUS.
• -Formed completely on 9th day of
incubation

9. • -Serves to digest the yolk and to
transfer the products of digestion to
the embryo.
• -Digestion by endodermal cells
• -At first distribution by diffusion , then
by vitelline veins and arteries on
development of AREA VASCULOSA.
• -As the yolk is digested, yolk sac
becomes small and before hatching is
withdrawn into body cavity.

13. YOLK SAC IN MAMMALS
• It act as yolk sac placenta in marsupials. In the
beginning yolk sac has a wide connection with the
midgut but later becomes narrow and called yolk stalk
or vitelline duct through which vitelline vessels pass.
• It is vestigeal in human beings. Its formation in absence of
yolk points to their reptilian ancestory. It first appears in the
form of trophoblast. Trophoblast develop villi which
absorbs uterine secretions temporarily till placenta is formed
after which it start degenerating.
• -It looses nutritive role early and then functions as
haemopoietic organ, a site of blood cells formation.

16. FUNCTIONS OF YOLK SAC
• Primary function is to act as yolk reservoir
and extra embryonic gut.
• It also helps in respiration in embryos of
reptiles and birds.
• In marsupials, yolk sac and chorion form
yolk sac placenta or chorio-vitelline.

18. CHICK

19. Amnion
-Amnion formation starts after 30 hrs of incubation
-Formed of somatopleur ( ectoderm and somatic mesoderm )
by folding
-Head & tail fold, amniotic umbilicus, seroamniotic
connection on 4th day
-Amnion , the inner arc of inner ectoderm & outer mesoderm
with amniotic cavity filled with amniotic fluid.
-It serves as a water cushion to protect the embryo from
mechanical injury, also neutralises the effects of changes in
external temperature, prevent embryo dessication.

21. CHORION
Chorion as outer arc with outer ectoderm &
inner mesoderm
Space between amnion and chorion is called
choriotic cavity containing fluid which
provides further protection to the embryo ,
also provides space for growth of Allantois.
Chorion later becomes pressed against the
shell membrane and plays important role in
gaseous exchange as allantochorion, the
lung and placenta in eutherians except
primates where only chorion participates in
placenta formation

22. • For formation a body fold is developed
from yolk sac
• -Tail fold grows more rapidly than head
fold,
Amnion & Chorion in mammals

23. -Absent in anamniotes as respiration and waste lost by diffusion in water
-Arises on third day of incubation, develops as a blind downgrowth from the floor
of hindgut.
-Formed of splanchnopleur ( splanchnic mesoderm & endoderm ) from ventral
side of hind gut
-Endoderm on inner side, splanchnic mesoderm on outer side
-Grows in choriotic cavity, fuses with somatic mesoderm of chorion and forms
allantochorion .It becomes highly vascularised, functions as extraembryonic
lung ,gaseous exchange taking place between the blood in it and external air
through shell.
-Allantois retains connection with the hindgut asAllantoic stalk. Later the
allantoic and yolk stalk form umbilical cord by being surrounded by common
body wall.
-
ALLANTOIS

27. FUNCTIONS OF ALLANTOIS
• Allantois serves to store nitrogenous waste matter, uric acid, outside the
embryo proper , so help in excretion and act as extra embryonic kidney.
During hatching allantois with nitrogenous wastes is left behind.
• In eutherians, allantois plays no excretory role as nitrogenous wastes of
embryo pass into maternal blood through placenta.
• Allantochorion in most eutherians except primates forms allantoic placenta.
• Also forms Allantoamnion which serves as a soft , elastic cushion protecting
the embryo from shocks.
• It is reduced in human beings.
• In most mammals it carries out excretion, respiration and nutrition.
• The base of allantois forms urinary bladder in amniotes except in birds.

37. PFA of two years 2015 and 2017.
http://www.mdurohtak.ac.in/pdf/Question_Booklets/Ent_exa
m_session_2015_16/M.Sc.%20Life
%20Sciences_ent_exam_2015_booklet.pdf
http://www.mdurohtak.ac.in/pdf17/admission2017_18/Ent_e
xam_ques_booklets/life%20sc%20booklet%20answer
%20key%20mducee2017.pdf